An airplane must have the following parameters for the flight controls:                the air speed indicated in two or three directions;        the speed of sound or the air temperature;        the static pressure.        
The indicated air speed is conventionally measured by the pressure difference observed between two precise points of a Pitot tube making it possible to maximize the aerodynamic pressure effect. It must be measured with an accuracy of the order of 15 cm/s.
The speed of sound is deduced from a temperature measurement called “total temperature” made at a point of aerodynamic compression of the atmosphere, usually at the nose of the aircraft. This speed is considerable in the field of transonic speeds. The accuracy of measurement is of the order of 1 m/s corresponding to a temperature difference of 2K.
The static pressure is measured at a point on the surface of the aircraft which is not very sensitive to aerodynamic differences. Its accuracy of measurement is approximately 20 Pa.
The indicated air speed is a combination of the real speed and the density of the air. It may therefore be measured based on a true air speed datum and on a measurement of the density of the atmosphere.
Currently, the probes necessary for measuring these parameters are extrusions to the structure of the cabin. They have many drawbacks. They are fragile, are sensitive to frost and require considerable defrosting means. They also generate additional drag.
It is possible to replace these probes with laser velocimeters which have the advantage of not being intrusive. The operating principle of a velocimetric probe is as follows. At a certain distance from the skin of the airplane, a network of interference fringes is created. When an atmospheric aerosol passes through this network, a modulated signal is sent back to the probe. Measuring the characteristics of the signal makes it possible to find the speed information that is sought. Nevertheless, this solution has certain drawbacks. It is not possible to measure the temperature by this means. Moreover, in the absence of aerosols, the measurement is no longer possible.